The Global Positioning System (GPS) has been used for many years to determined position and attitude of a moving vehicle. Such a system is particularly difficult to implement on a space vehicle due to its rotational spin. For example, an antenna mounted on the fuselage will experience "shadowing" wherein the vehicle body periodically blocks reception of a GPS satellite signal.
The most common GPS attitude-sensing system is based on short base line interferometric processing techniques. It employs a pair of spaced-apart antennas receiving signals from the same GPS satellites. The antennas are connected to conventional GPS receivers which sense the GPS carrier phases at the antennas. The attitude of the vehicle can be readily calculated from the phase differences and the inter-antenna distance.
As an alternative to the multi-antenna systems, another prior system utilizes a single rotating antenna mounted at an offset from the spin axis of a rotating vehicle. The rotating antenna introduces a sine wave modulation on the "normal" carrier phase measurements. The frequency of the modulation corresponds to the rotational rate of the vehicle; the amplitude of the modulation corresponds to the angle of incidence between the direction of the GPS satellite and the rotational plain of the vehicle; and the phase of the modulation corresponds to the satellite azimuth in the rotational plane.
These prior systems have significant constraints which render them unsuitable for use on a vehicle with a high rotational rate. Specifically, they require that the receivers maintain "lock" with a minimum of four satellites for extended periods. In addition, these systems require that their antennas simultaneously view all satellites used and this is impossible on a rotating vehicle. Space vehicles also present special challenges for GPS receiver tracking technology. The relatively high spin rate of a space vehicle, when combined with the large fuselage diameter, may produce a very large acceleration dynamic on tracking loops. Linear velocity, acceleration and jerk (change of acceleration) caused by the activation and deactivation of booster stages also produce dynamics that significantly impact tracking loop performance.
What is needed, therefore, is an improved method for tracking GPS satellite signals to accurately determine the position, acceleration, attitude and rotational rate of a space vehicle having a high rotational speed. The method must be capable of operating without continuous signal power and must employ wide noise bandwidth tracking loops to handle vehicle dynamics.